maybe you have disk errors, run scandisk:
open "My Computer" > Right click "local disk (C:)" and select "Properties>Tools>error-checking>start now"
with all other programs closed ;)

don't enable "fix errors" this time. But if it finds any errors, try to save any bookmarks, media and personal files and settings to floppy/CD-R before running diskscan with fix errors enabled, just in case you need to reformat.